The present invention is related in general to the field of semiconductor devices and processes and more specifically to the materials and fabrication of leadframes for integrated circuit devices.
The leadframe for semiconductor devices was invented (U.S. Pat. No. 3,716,764 and No. 4,034,027) to serve several needs of semiconductor devices and their operation simultaneously: First of all, the leadframe provides a stable support pad for firmly positioning the semiconductor chip, usually an integrated circuit (IC) chip. Since the leadframe including the pads is made of electrically conductive material, the pad may be biased, when needed, to any electrical potential required by the network involving the semiconductor device, especially the ground potential.
Secondly, the leadframe offers a plurality of conductive segments to bring various electrical conductors into close proximity of the chip. The remaining gap between the (xe2x80x9cinnerxe2x80x9d) tip of the segments and the conductor pads on the IC surface are typically bridged by thin metallic wires, individually bonded to the IC contact pads and the leadframe segments. Obviously, the technique of wire bonding implies that reliable welds can be formed at the (inner) segment tips.
Thirdly, the ends of the lead segment remote from the IC chip (xe2x80x9couterxe2x80x9d tips) need to be electrically and mechanically connected to xe2x80x9cother partsxe2x80x9d or the xe2x80x9coutside worldxe2x80x9d, for instance to assembly printed circuit boards. In the overwhelming majority of electronic applications, this attachment is performed by soldering. Obviously, the technique of soldering implies. that reliable wetting and solder contact can be performed at the (outer) segment tips.
It has been common practice to manufacture single piece leadframes from thin (about 120 to 250 xcexcm) sheets of metal. For reasons of easy manufacturing, the commonly selected starting metals are copper, copper alloys, iron-nickel alloys for instance the so-called xe2x80x9cAlloy 42xe2x80x9d), and invar. The desired shape of the leadframe is etched or stamped from the original sheet. In this manner, an individual segment of the leadframe takes the form of a thin metallic strip with its. particular geometric shape determined by the design. For most purposes, the length of a typical segment is considerably longer than its width.
In the European patent # 0 335 608 B1, issued Jun. 14, 1995 (Abbott, xe2x80x9cLeadframe with Reduced Corrosionxe2x80x9d), a palladium-plated leadframe is introduced which is not subject to corrosion due to galvanic potential forces aiding the migration of the base metal ions to the top surface where they will form corrosion products. The patent describes a sequence of layers consisting of nickel (over the base metal), palladium/nickel alloy, nickel, and palladium (outermost). This technology has been widely accepted by the semiconductor industry for copper-based leadframes.
After assembly on the leadframe, most ICs are encapsulated, commonly by plastic material in a molding process. It is essential that the molding compound, usually an epoxy-based thermoset compound, has good adhesion to the leadframe and the device parts it encapsulates. Palladium, described above as the outermost layer of the leadframe, offers excellent adhesion to molding compounds.
Unfortunately, palladium is expensive; its price limbed in the last decade from about one third of the gold rice to about 20% higher than gold. Cost reduction pressures in semiconductor manufacturing have initiated efforts to reduce the thickness of the palladium layers employed to about one third of its previous thickness. At this thinness, palladium does not prevent oxidation of the underlying nickel which will inhibit its solderability. A method introduced in semiconductor manufacturing uses a thin layer of gold on the palladium surface to prevent oxidation. One related example is described in U.S. Pat. No. 5,859,471, issued on Jan. 12, 1999 (Kuraishi et al., xe2x80x9cSemiconductor Device having TAB Tape Leadframe with Reinforced Outer Leadsxe2x80x9d).
In these methods, however, the entire surfaces of the leadframe are plated with gold. This practice severely inhibits the adhesion of the leadframe segments to molding compounds and risks delamination in thermomechanical stress testing. Furthermore, the plating of the complete leadframe with a thin gold layer makes it impossible to decide by visual inspection whether a leadframe has the gold surface or not. Such standard simple inspection, however, is highly desirable as manufacturing practice. Finally, the deposition of gold in unnecessary areas is counterproductive to cost saving efforts.
A more cost-effective method of gold-spot plating is described in U.S. Patent Application No. 60/125,304, filed on Mar. 19, 1999 (Abbott et al., xe2x80x9cGold Spot Plated Leadframes for Semiconductor Devices and Method of Fabricationxe2x80x9d). It is till based, though, on the technology of preplating and depositing layers of precious metal onto the metal base, usually copper. Consequently, leadframes using this method and related techniques of preplating will remain a high-cost part in semiconductor packages.
An urgent need has therefore arisen for a radically new low-cost, reliable mass production method for semiconductor leadframesxe2x80x94especially for the widely accepted copper leadframesxe2x80x94which provides all the assembly features leadframes are expected to offer: Bondability, solderability, and adhesion to polymeric compounds. The new leadframe and its method of fabrication should be flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations, and should achieve improvements toward the goals of improved process yields and device reliability. Preferably, these innovations should be accomplished using the installed equipment base so that no investment in new manufacturing machines is needed.
According to the present invention for a semiconductor integrated circuit (IC) leadframe, the base metal of the leadframe has a modified surface, comprising a layer created by converting a percentage of the base metal atoms into substitutional metal complexes and adapted to provide bondability and solderability and adhesion to polymeric compounds. The leadframe is fabricated by first cleaning and activating the leadframe surface, and then immersing the activated leadframe into a chromating solution containing chromic acid and an activator, thereby converting surface atoms of the base metal into chromate complexes and creating a surface layer comprising chromic and base metal reaction products. The technique is especially applicable to copper-based leadframes. For controlling uniformity and consistency, a layer of base metal material may be plated onto the cleaned leadframe before proceeding with the surface activation step.
The present invention is related to high density ICs, especially those having high numbers of inputs/outputs., and also to low end, low cost devices. These ICs can be found in many semiconductor . device families such as standard linear and logic products, digital signal processors, microprocessors, digital and analog devices, high frequency and high power devices, and both large and small area chip categories. The package type can be plastic dual in-line packages (PDIPs), small outline ICs (SOICs), quad flat packs QFPs), thin QFPs (TQFPs), SSOPs, TSSOPs, TVSOPs, and other leadframe-based packages.
Since there is no nickel, silver, palladium, or gold in the leadframe of the finished package, the invention represents a significant cost reduction of the semiconductor packages, especially the plastic molded packages, compared to the conventional copper-based nickel-palladium-plated leadframes.
The theory underlying the surface conversion of a base metal can, for example, be found in xe2x80x9cMetal Finishingxe2x80x9d, Guidebook and Directory Issue 98, January 1998, volume 95, number 1; published by Metal Finishing, 660 White Plains Ave., Tarrytown, N.Y. 10591. xe2x80x9cChromate conversion coatings are chemical conversion coatings. The substrate metal participates in the coating reactions and becomes a component of the coating.xe2x80x9d xe2x80x9cFilm formationxe2x80x94the films in most common use are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of other components, or xe2x80x98activatorsxe2x80x99, in an acid solution. The hexavalent chromium is partially reduced to trivalent chromium during the reaction, with a concurrent rise in pH, forming a complex mixture consisting largely of hydrated basic chromium chromate and hydrous oxides of both chromium and the basis metal. The composition of the film is rather indefinite, because it contains varying quantities of the reactants, reaction products and water of hydration, as well as the associated ions of the particular systems.xe2x80x9d
It is an aspect of the invention to provide a technology for modifying the surfaces of copper or copper-alloy-based leadframes to provide bondability, solderability and adhesion to polymeric die-attach and molding compounds, without requiring costly fabrication processes.
The bondability is compatible with the conditions defined by fine-pitch bonding. The solderability conforms with surface mount technologies based on bending the package leads and solder attaching lead portions without flux (or only very mild fluxes not requiring rinse steps). The end user of the semiconductor product can continue to employ the installed assembly equipment base. The excellent adhesion to die attach and molding compounds used on plastic packages prevents delamination, moisture ingress, and corrosion.
Another aspect of the invention is to reach these goals without the cost of equipment changes and new capital investment and using the installed fabrication equipment base. The alkaline elctroclean and flood and soak baths are standard; the mass production processes are based on reel-to-reel manufacturing. The line is very short and the process very fast.
Another aspect of the present invention is the option of modifying the fabrication process by depositing a plated layer of copper onto the cleaned leadframe. This modification provides controlled uniformity and consistency to the replacement reaction and the conversion to substitutional metal complexes.
Another aspect of the invention is the standard appearance of the finished product. For copper or copper-plated leadframes according to the invention, the leadframe looks coppery with a light gray cast to it, and as a finished device, the external leads look similar to the leadframe.
These aspects have been achieved by the teachings of the present invention concerning surface atoms replacement reactions. Various modifications of leadframe preparations have been successfully employed.
In the first preferred embodiment of the invention, strip-shaped leadframes, made of copper or copper alloys, are processed in reel-to-reel fabrication equipment. After proceeding through alkaline electro-clean and activation baths, the leadframes are exposed to a non-electrolytic chromate conversion reaction, in which copper surface atoms are replaced by a mixture of chromium, hydrous oxide and copper complexes. Appropriate rinsing steps are inserted.
In the second preferred embodiment of the invention, strip-shaped leadframes, made of copper or copper alloys, are processed in reel-to-reel fabrication equipment. After proceeding through an alkaline electro-clean bath, the leadframes receive a plated layer of pure copper in controlled grain structure, uniformity and consistency. The subsequent process steps of activation and chromation are the same as in the first preferred embodiment.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.